The ATCRBS/SIF system presently in use employs ground based interrogator transmitters which query airborne transponders within the range of operation of each. Associated with each interrogator are a reply receiver, quantizer, reply decoder, and reply processor which together process received replies of the airborne transponders. These replies contain informational pulses which may identify the aircraft, convey altitude information or convey other data, depending upon interrogation coding. Traffic Alert and Collision Avoidance Systems (TCAS) of the "active" (interrogating) type include similar airborne interrogators and transponders to intercommunicate with other aircraft. Several pulse-coding modes are in use for interrogation and reply signals; the present invention accommodates all presently used modes.
A large number of beacon interrogators are in operation in many metropolitan areas. Typically, large numbers of aircraft are within operational range of one or more or these interrogators. Consequently, replies from several aircraft will often be received simultaneously by each interrogator station. These received replies are of two types; valid and false. Only those replies which are valid responses to a particular interrogation are of interest to the respective interrogator. Other replies, known as False Replies Unsynchronized In Time (FRUIT), cause a major processing problem which becomes acute in high reply density areas.
Hardware means are included in reply decoders and software means are included in reply processors at the interrogation stations to minimize effects of such FRUIT when "garbled"replies exist, i.e., where two or more replies arrive at the interrogator receiver at approximately the same time. Detection and degarbling of overlapping valid replies is a second major problem confronted by reply decoders. It is desirable to eliminate reply garbling without losing valid replies.
The reply formats prescribed for the ATCRBS/SIF reply modes include one leading and one trailing framing pulse separated by 20.3 microseconds. The trailing pulse may be followed by a Special Position Identification (SPI) pulse, or by subsequent pairs of framing pulses at a 4.35 microsecond interval. Valid reply pulse trains are recognized by this spacing between framing pulses, and informational pulses are synchronized for decoding based upon the time of the initial framing pulse. This function is accomplished by a bracket decoder circuit on a digital delay line in the reply decoder. The reply format gives rise to the third problem plaguing reply decoders and processors when overlapped or closely spaced replies are present. Replies known as "phantom replies" occur whenever two framing or informational pulses arrive at the reply decoder with the same time separation between them (20.3 microseconds), as two valid framing pulses. Obviously, it is desirable to identify and discriminate against phantom replies while saving those valid informational pulses which may have been complicit in formating of the phantom.